C library usage is not conditional upon architecture

These were conditioned on architecture because traditionally MIPS was
dynamically linked, while other arches used musl for static linking. Now
that we support both methods of linking on all architectures, these
special cases no longer make sense

Add a comment explaining why copying startup files is always necessary

Author: smaeul

src/bootstrap/compile.rs

@@ -112,14 +112,17 @@ pub fn std_link(build: &Build,
     t!(fs::create_dir_all(&libdir));
     add_to_sysroot(&out_dir, &libdir);
 
-    if target.contains("musl") && !target.contains("mips") {
+    if target.contains("musl") {
         copy_musl_third_party_objects(build, target, &libdir);
     }
 }
 
 /// Copies the crt(1,i,n).o startup objects
 ///
-/// Only required for musl targets that statically link to libc
+/// Since musl supports fully static linking, we can cross link for it even
+/// with a glibc-targeting toolchain, given we have the appropriate startup
+/// files. As those shipped with glibc won't work, copy the ones provided by
+/// musl so we have them on linux-gnu hosts.
 fn copy_musl_third_party_objects(build: &Build, target: &str, into: &Path) {
     for &obj in &["crt1.o", "crti.o", "crtn.o"] {
         copy(&build.musl_root(target).unwrap().join("lib").join(obj), &into.join(obj));

src/bootstrap/sanity.rs

@@ -157,8 +157,8 @@ pub fn check(build: &mut Build) {
             panic!("the iOS target is only supported on macOS");
         }
 
-        // Make sure musl-root is valid if specified
-        if target.contains("musl") && !target.contains("mips") {
+        // Make sure musl-root is valid
+        if target.contains("musl") {
             match build.musl_root(target) {
                 Some(root) => {
                     if fs::metadata(root.join("lib/libc.a")).is_err() {

src/libunwind/build.rs

@@ -15,7 +15,7 @@ fn main() {
     let target = env::var("TARGET").expect("TARGET was not set");
 
     if target.contains("linux") {
-        if target.contains("musl") && !target.contains("mips") {
+        if target.contains("musl") {
             // musl is handled in lib.rs
         } else if !target.contains("android") {
             println!("cargo:rustc-link-lib=gcc_s");